Abstract: Aims Populations are the basis for the formation and development of the structure and function of grassland ecosystems. However, long-term grazing accompanied by global climate change profoundly affects the growth and reproduction of populations. Stipa bungeana is a dominant species in typical steppe of the Loess Plateau and has a high ecological and economic value. Methods Therefore, this study was conducted to investigate the effects of grazing and nitrogen (N) addition on the growth of Stipa bungeana. It was based on the platform of a long-term rotational sheep grazing experiment in a typical steppe of the Loess Plateau, using a completely randomized split-plot experimental design with stocking rate (0, 2.7, 5.3, 8.7 sheep·hm–2) as the main factor and N addition levels (0, 5, 10, 20 g·m–2) as the secondary factor. Morphological traits, aboveground biomass, community status and the relationship between them in Stipa bungeana were used to investigate the effects of stocking rate, nitrogen (N) addition and their interaction. Important findings With the increase of stocking rate, the plant height, canopy diameter, tiller number, seedlings, aboveground biomass and ration of aboveground biomass showed a “single peak” curve trend, and the population density decreased. nitrogen (N) addition increased the plant height, canopy diameter, reproductive branch density, tiller number, aboveground biomass and ratio of aboveground biomass, the seedlings increased and then decreased with the increase of nitrogen (N) addition. The total effect of grazing on aboveground biomass and ratio of aboveground biomass was small compared with that of nitrogen (N) addition. In concrete terms, grazing had a direct negative effect on aboveground biomass and affected its ratio of aboveground biomass by regulating tiller number, population density and aboveground biomass. nitrogen (N) addition not only had a direct positive effect on aboveground biomass, but also had an indirect positive effect through plant height, reproductive branch density, and also had an impact on ratio of aboveground biomass through regulating population density, canopy diameter, tiller density and reproductive branch density. Overall, nitrogen (N) addition increased canopy diameter and reproductive branch density and grazing increased seeding, while the interaction of grazing and nitrogen (N) addition significantly affected reproductive branch density. Stipa bungeana had optimal aboveground biomass or community status at stocking rate of 4.10 sheep?hm-2 or 5.29 sheep?hm-2. The results indicated that grazing and nitrogen (N) addition regulated the aboveground biomass and community status of Stipa bungeana through influencing it's morphological characteristics, which provides a basis for the scientific management and sustainable development of grassland populations.

Key words: grazing, nitrogen (N) addition, population, morphological traits, aboveground biomass, community status